Prenominal adjective order is such a fat big deal because adjectives are ordered by likely need.

When multiple adjectives precede a noun in English, they are often ordered in a way that is implicitly understood by all fluent speakers of the language. Adjective order might be described as a big fat deal, but to describe it as a fat big deal betrays a lack of knowledge of English. Sweet (A New English Grammar: Part II, 1898/1955) proposed two related semantic principles to explain the phenomenon: definiteness of denotation (adjectives that denote a property that is most independent of the modified noun must be placed furthest from that noun) and closeness of adjective/noun in meaning (adjectives that denote properties essential to or inherent in the modified noun are placed closer to the noun). These observational descriptions of the phenomenon have received experimental support (Martin, Journal of Verbal Learning and Verbal Behavior, 8(6), 697-704, 1969). However, the issue of why Sweet's rules are true has not yet been solved. I propose, operationalize, test, and find strong support for a simple theory: that prenominal adjective order reflects likely need, the a priori probability that a particular adjective will be needed.